In recent years, with improved luminous efficiency and lower prices of LEDs, illumination devices having LEDs as a light source have become popular. Of the illumination devices using an LED light source, as an illumination device to be mounted and used on a ceiling and the like of an office or a residence and the like, in particular, a direct-underneath type illumination device which has LEDs arranged as dots in a grid or radially on the plane on the side of a bottom surface opposed to an emission surface of this illumination device has been popular.
However, since LEDs are arranged as dots directly on a bottom surface in the direct-underneath type illumination device, the illumination device cannot only be looked directly because of its strong glare but also lacks uniformity of brightness as a planar luminous body. Although a milky diffuser plate and the like are used as a cover for the illumination device to alleviate these problems, it reduces utilization efficiency of light.
On the one hand, an edge-light type has been known as a backlight for a liquid crystal display device in a liquid crystal television or a personal computer and the like. A transparent plate-shaped light-guide plate made of a transparent polymer such as PMMA (polymethyl methacrylate) or the like is installed in an edge-light type backlight.
White dots (scattering dots) are printed on a bottom surface of this light-guide plate. Through adjustment of size, density and the like of the dots, a part of light propagating in the light-guide plate is scattered when hitting the white dots, and emitted in a visible (emission surface) direction.
The edge-light type backlight can illuminate with planar light by receiving, into a light-guide plane, light from a primary light source (LEDs or the like) arranged on at least one side face of a light-guide plate and emitting light from one entire main surface (emission surface). Since the primary light source is not looked directly in the edge-light type, it has no glare and is excellent in the quality of appearance.
Thus, this edge-light type backlight has been applied to lighting equipment which is mounted and used on a ceiling of an office or a residence and the like (See Patent Literature 1, for example). Behind the background, thinning of the lighting equipment with an LED as a light source becomes possible due to improved luminous efficiency and lower prices of LEDs, thereby enabling weight reduction of illumination devices. In addition to this, since dimming control, which is a feature of an LED, is possible, the edge-light type illumination device with the LED as a light source has become popular.